1. Field of the Invention
The present invention relates to a position detecting apparatus and an abnormality detecting apparatus, and more particularly to a resolver for detecting a position or a device for detecting the rotational position such as a synchro, and most particularly to a position detecting apparatus and an abnormality detecting apparatus for detecting the position of a motor for driving a brushless motor.
2. Description of the Related Art
FIG. 7 shows a conventional position detecting apparatus of this type as disclosed in, for example, Japanese Patent Laid-open No. 9-126809. Referring to FIG. 7, for example, a position sensor 10 made up of a resolver is inputted with an excitation signal (sin xcfx89t) which is a sine wave voltage for excitation from an oscillating circuit 22 that generates the excitation signal on the basis of a clock of a counter 12. The position sensor 10 outputs induction signals sin xcex8 sin(xcfx89txc2x1xcex1) and cos xcex8 sin(xcfx89txc2x1xcex1) resulting from amplitude-modulating the excitation signal cos xcex8 and sin xcex8 in response to the rotational position xcex8. Here, xcex1 represents a phase fluctuation error of the output signal to the excitation signal due to a path of from the position sensor 10 to a position detecting circuit or a change in the temperature of a position sensor winding.
One output signal sin xcex8 sin(xcfx89txc2x1xcex1) has its phase shifted by a phase shifting circuit 14 by a given value of 90 degrees, so the output signal becomes a phase shift output sin xcex8 cos(xcfx89txc2x1xcex1). The phase shift signal and another output signal cos xcex8 sin(xcfx89txc2x1xcex1) from the position sensor 10 are added together by an adding circuit to output an addition signal sin(xcfx89t+xcex8xc2x1xcex1). Similarly, the phase shift signal and another output signal cos xcex8 sin(xcfx89txc2x1xcex1) outputted from the position sensor 10 are subtracted from each other by a subtracting circuit to output a subtraction signal sin(xcfx89txe2x88x92xcex8xc2x1xcex1).
The excitation signal (sinxcfx89t) which is a reference signal is counted with reference to a counter 12. That is, since the excitation signal is counted as one period of from 0 to 360 degrees, a zero-cross point of the addition signal sin(xcfx89t+xcex8xc2x1xcex1) is detected by a zero-cross detecting circuit 17, and data D1 latched by a latch circuit 19 becomes +xcex8xc2x1xcex1. Similarly, the zero-cross point of the subtraction signal sin(xcfx89txe2x88x92xcex8xc2x1xcex1) is detected by the zero-cross detecting circuit 18, and data D2 latched by the latch circuit 20 becomes xe2x88x92xcex8xc2x1xcex1. In addition, in an error calculating circuit 21, (D1+D2)/2 becomes xc2x1xcex1 as described above. Therefore, the phase fluctuation error xc2x1xcex1 can be calculated. The data D1 and the phase fluctuation error xc2x1xcex1 are subtracted from each other by the subtracting circuit 22, thereby being capable of obtaining xcex8, and also capable of extracting the position xcex8 from which the phase fluctuation error xc2x1xcex1 is removed.
Because the conventional position detecting apparatus disclosed in Japanese Patent Laid-Open No. 9-126809 is structured as described above, when the position detecting apparatus fails due to some cause, in particular when the phase shifting circuit 14 fails and the phase shift is not effected, the addition signal becomes sin xcex8 sin(xcfx89txc2x1xcex1)+ cos xcex8 sin(xcfx89txc2x1xcex1)=(sin xcex8+cos xcex8)sin(xcfx89txc2x1xcex1), and the subtraction signal becomes sin xcex8 sin(xcfx89txc2x1xcex1)xe2x88x92cos xcex8 sin(xcfx89txc2x1xcex1)=(sin xcex8xe2x88x92cos xcex8)sin(xcfx89txc2x1xcex1). Therefore, the data D1 becomes xc2x1xcex1 or 180 degrees xc2x1xcex1 except that xcex8 is 135 degrees or 315 degrees, and also the data D2 becomes xc2x1xcex1 or 180 degrees xc2x1xcex1 except that xcex8 is 45 degrees or 225 degrees. Therefore, xcex8 detects any one of xc2x1xcex1, 90 degrees xc2x1xcex1, 180 degrees xc2x1xcex1, and 270 degrees xc2x1xcex1. For simple description, if xcex1 is 0, a relationship between the original position (for example, motor rotation position) and the detected position is shown in FIG. 8. The error becomes 135 degrees at the maximum, and in the case where the apparatus is used as a motor rotation position detecting apparatus for driving a brushless synchronous machine, there is the possibility that malfunction occurs such that the apparatus drives the synchronous machine in a direction opposite to a normal drive direction. In order to prevent this drawback, there has been proposed that two position detecting apparatuses are disposed, and two detection results are compared with each other to judge whether a failure occurs, or not. In this proposal, since two position detecting apparatuses are required, it becomes expensive.
Also, in order to calculate the position xcex8, because the phase fluctuation error xc2x1xcex1 is calculated once, and then calculation processing except for the position fluctuation error is required, the structural circuit becomes complicated and expensive.
The present invention has been made under the above circumstances, and therefore an object of the present invention is to provide a position detecting apparatus and an abnormality detecting apparatus which are capable of minimizing an influence of a failure even if a phase shifting circuit fails.
With the above objects in view, the position detecting apparatus comprises: a position sensor that inputs an excitation signal having a given period waveform and outputs a first amplitude modified signal induced in accordance with a detected position from a first output winding and outputs a second amplitude modified signal induced in accordance with a detected position from a second output winding; a first phase shifting means for electrically shifting a phase of the first amplitude modified signal by a given angle; a second phase shifting means for electrically shifting a phase of the second amplitude modified signal by a given angle; a converting means for converting the signal outputted from the first phase shifting means and the signal outputted from the second phase shifting means into a phase modulated signal; and a calculating means for obtaining a detected position on the basis of the phase of the phase modulated signal outputted from the converting means.
The calculating means may detect a phase difference between the excitation signal and the phase modulated signal to obtain the detected position.